Fix an issue where the pointer could be invisible

Add setBackgroundAlpha and use that to change the alpha of the
expanded view background and pointer.

Test: manual - have a bunch of bubbles, expand them, dismiss the
               selected one
               => verify that the arrow is visible for the next bubble
	     - turn on home animation, swipe up to collapse a bubble
               => verify pointer looks fine
Bug: 234739591
Change-Id: If5823840d148e2031312c54eba9c0132b8aa3a4c